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Do batteries contain a “buffer”? My kW usage doesn’t match pack size

Discussion in 'Model 3: Battery & Charging' started by lateulade, Oct 1, 2019.

  1. lateulade

    lateulade Member

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    I have a SR (downgraded from SR+ after 15k miles) and I kill time on long toad trips by doing mental math. The EPA reported capacity of the SR+ was 54.5kWh, but every single measurement I could come up with (pack size/rated mileage, actual consumption/mileage, consumption as a % or SOC etc) indicated that I had 50kWh of useable energy.

    Since downgrading to SR, it seems like I have ~45kWh rather than 50.

    Is there something wrong with my pack, or is there a ~10% “buffer” not represented by the SOC% display?
     
  2. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    #2 AlanSubie4Life, Oct 1, 2019
    Last edited: Oct 1, 2019
    There is nothing wrong. There is some buffer but it is not 10% (it's more like 3%). I am not sure exactly what your pack capacity should be for SR but it is probably something like:

    220rmi * 219Wh/rmi = 48.2kWh

    (rmi = rated mile)

    There's a discussion on this page (read the 5-10 preceding posts as well ;) ):

    Empirical Trip Meter Wh/mi constants

    The trip meter is not representative of your battery capacity. What you should look at is the charging screen for a charge cycle - say you add 150mi to your pack, you will see (if you switch to %) that that corresponds to 33kWh (32.85kWh = 150rmi*219Wh/rmi ).

    So you can extrapolate that to 32.85kWh * 220rmi/150rmi = 48.2kWh (or 52.56kWh for the SR+)

    But beyond that, there is some reserve energy (if the BMS/Soc estimation is working correctly) below 0 miles. It's 2-3kWh. So add that and you're at the ~54.5kWh.


    The trip meter, for the SR+ (and probably the SR), on the other hand, apparently will show ~209Wh per rated mile used (not per mile traveled!!!). (I've never verified this myself since I have a Performance, but see that other thread.) So for an SR you'd probably see it extrapolate to a discharge from 220rmi to 0rmi as 220rmi * 209Wh/rmi = 46kWh.

    Note also a little subtlety: the EPA discharged the battery to dead for its tests (not to 0 rated miles). So actually the EPA rated miles aren't the same as the rated miles on the battery gauge I refer to above (the rated miles on the battery gauge above are slightly lower energy content than the EPA rated miles, by approximately the % of reserve capacity)

    If these numbers don't match your mental math on your next road trip, do check back in here with any corrections. (Obviously the trip meter does not take into account battery use when in Park, so be sure to avoid that state if you're doing math.)
     
    • Informative x 1
  3. Rocky_H

    Rocky_H Well-Known Member

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    This is intentional. Here's the thing:
    With lithium ion batteries, if they actually discharge down to no energy left, with 0V, they are destroyed and cannot be recharged ever again. So with all electric cars, they build in what is nicknamed the "anti-bricking buffer". It prevents you and I from accidentally murdering our batteries. With Tesla cars, this has generally been about 5 kWh. So congratulations, you have figured out about how big that is on your own! So that is built into the behavior of the car's software, that if it gets down to that level of 0 "rated miles" left, and the anti-bricking reserve level is all that is in there, it will shut the car down immediately to prevent the batteries from getting ruined.
     
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  4. ASUComputerGuy

    ASUComputerGuy Supporting Member

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    I recently did this and came to the realization that the climate control (A/C) usage is not calculated in your energy usage in the trip screens. I figured for my MS, the A/C uses about 2kW of power, so for every hour I'm in the car on a trip, I have to calculate an additional 2kWh of battery for that. The heat will use much more; I believe the last I heard was heat uses 6kW. Maybe this is where your missing energy went to???
     
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  5. Rocky_H

    Rocky_H Well-Known Member

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    No, that's false. What you are probably seeing or thinking of is that those trip screens do not accumulate the energy use while the car is parked or turned off. It definitely does count all energy usage, including all heat and air conditioning while the vehicle is in Drive.
     
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  6. TimothyHW3

    TimothyHW3 Member

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    Yes, Tesla's have buffers. The 100% estimate includes the buffer in the calculation, but you only get to "use" it if you drop below 0%( don't try it, but you can still drive slowly for about 3.5kWh of battery).
    The buffer on LR is about 3.5kWh with total 77kWh and I assume it will be the same on SR+, 3-3.5kWh. So you have about 50kWh of which you can use about 80-90% unless you want to kill the battery. When Tesla downgraded you they changed the BMS so now you have around 45kWh, of which, once again, you should only use 80-90%. Also, driving at higher speeds will result in heat loss and if you drive slow speeds with the AC, this will also affect your rated miles. Check my account and read my other posts so that I don't repeat myself?
     
  7. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    This is not quite correct. It's closer to 220/240 * 54.5kWh = 49.9kWh. The ~54.5kWh is from the SR+ EPA document from my recollection (and also quoted above). (EPA documents for the LR say about 78kWh; in agreement with your ~77kWh number.)
     
  8. TimothyHW3

    TimothyHW3 Member

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    I don't have the SR/SR+ numbers from CAN as these are fairly new cars in Europe. The LR has exactly 77kWh brand new is from the data we have ,(5 cars). I go from what Bjørn Nyland measured a on SR+(49-50kWh) and the EPA(if they say 78 on LR and we have 77, I assume the same thing with SR+55-54)
    But wether it is 49 or 50 , or 44-45 on SR doesn't really matter.

    Hopefully we will have SR+ data soon.
     
  9. earthwormjim

    earthwormjim Member

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    The EPA number is a measurement at the power outlet feeding the car's charger. It's not a measurement of battery capacity, merely how much energy is consumed by the batteries' charge controller to fully charge the batteries. There are charging losses, roughly 5-10%, so 54kwh consumption to charge the batteries, equates to roughly a 50kwh battery capacity after accounting for losses.
     
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  10. AlanSubie4Life

    AlanSubie4Life Efficiency Obsessed Member

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    The EPA Wh/100mi number indeed does include charging losses. However, that is not the topic of discussion here.

    If you look at the EPA document you will be able to see separate numbers for the discharge energy (about 54.4kWh) and the charging event (something like 62kWh - do not quote me on that number - but it is in the document (which is linked elsewhere here)).
     
    • Informative x 1
  11. earthwormjim

    earthwormjim Member

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    You're correct, it was 61.19 kwh to recharge the battery according to the EPA file.

    Based on published energy density of the battery, it works out to 54.4 kwh. 378 kg, 144 wh/kg.

    https://iaspub.epa.gov/otaqpub/display_file.jsp?docid=46584&flag=1
     
  12. Eagledon

    Eagledon Member

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    I may be of some help. I have my Model 3 SR+ for all of 2 weeks and today was my first real road trip but if my calculations are correct using a data file I can download from Smartcharge rewards program I get the usable battery capacity at 49.38 kw/h.
    Sharing the file I made no attempt to clean it up though so use it for what it is worth.
    2019-10-01_2019-11-01 (Trips)
     

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